1. Field of the Invention
The present invention relates to a direction changing flow duct, for example, a compressor inlet duct for a turbocharger, wherein the direction of flow has to change abruptly in a short distance.
2. Description of the Related Art
Turbochargers are widely used on internal combustion engines, and in the past have been particularly associated commercially with large diesel engines, especially on-highway trucks, agriculture, off-highway and marine applications. Turbochargers are becoming more common on gasoline powered automobiles and are required on Diesel automobiles to meet emissions regulations. Engine exhaust gases are directed to and drive a turbine, and the turbine shaft is connected to and drives the compressor. Ambient air is drawn through a filter and then into, and compressed by, the turbocharger compressor and fed into the intake manifold of the engine. The design and function of turbochargers are described in detail in the prior art, for example, U.S. Pat. Nos. 4,705,463; 5,399,064; and 6,164,931, the disclosures of which are incorporated herein by reference.
Turbocharged vehicles are required to meet increasingly stringent emissions standards. Engines are being provided with auxiliary systems to accomplish this and other objectives, which systems take up space in the engine compartment. In this environment, it is a common problem that space in the engine compartment is limited. It is also standard that all air supplied to a turbocharger compressor must first pass through an air filter to remove any particulate matter that might damage the turbocharger and/or the engine. Due to space limitations, the air filter and compressor components frequently cannot be located close together in the engine compartment, and in these ducts the air filter is connected to the compressor via a an intake duct.
FIG. 1 depicts a typical, straight six, commercial Diesel engine installation, in a truck. The engine block (1) usually straddles the front axle (2) to provide desired weight distribution over said axle. It is natural, from an exhaust flow perspective, to have the turbocharger mounted centrally on the exhaust manifold (12). This mounting position means that the turbocharger is often adjacent to the front wheel (5) and the vehicle suspension, thus creating a side-to-side space constraint. The air filter (6) is positioned to limit the compromise between airflow from the vehicle to the filter (6) and the length of the duct (61) from the filter (6) to the compressor cover (20). A product of this compromise is the distance from the compressor cover (20) to the engine ancillaries such as the alternator (7) and the air conditioner compressor (8). These engine ancillary devices must be positioned at the front of the engine as they are often driven off the serpentine belt (9), which is driven by a pulley mounted on the nose of the engine crankshaft.
The compressor must be configured such that the duct (21) connecting the compressor cover (20) to the vehicle intercooler (6) has a clear run. The turbocharger must also be positioned such that the turbine stage (10) of the turbocharger is such that the exhaust pipe (11) has a reasonable run to the back of the vehicle. The exhaust pipe has to snake its way from the turbocharger, around the chassis rail (4), avoiding items which may be negatively affected by the exhaust temperature, like fuel and air tanks, tires etc. The exhaust pipe is usually 10% to 20% larger than the compressor discharge or inlet, so combined with the temperature of the pipe, the degree of difficulty in determining an appropriate, three dimensional route for the pipe means that this aspect of the vehicle installation design tends to take predominance over the compressor inlet ducting. The resultant of these configuration compromises is often that the compressor inlet is very close to other engine components.
Since the compressor inlet is often located near these obstructions, the air intake duct (61) from the air filter (6) to the compressor (20) has to be squeezed in around these components. Since the air filter is usually located either adjacent to the front of the engine, or on the vehicle firewall adjacent to the rear of the engine, the compressor-end of the duct from the air filter is frequently oriented perpendicular to the turbocharger centerline. In some off highway installations the exhaust discharges to the front of the vehicle so the turbocharger is reversed and the air inlet system is usually in conflict with a separate set of obstacles. In any case the inlet to the compressor is usually the last thought in the design and as a consequence it is often found to be lacking, from the turbocharger aerodynamic sense.
To one well versed in the art it is common knowledge that a rule of thumb is to allow a length of five diameters after a bend or direction changing modification to a duct, before introducing the flow downstream to a device. Such direction changing bends or elbows are well known. Also well known are the deficiencies inherent in such direction changing ducts, which were tested by the inventors to confirm the results of such bends.
First, at the inlet to the duct the pressure gradient at any point in the plane perpendicular to the centerline of the duct is small usually due to the length of the duct. Downstream of the bend in the duct the pressure gradient in the plane shown in FIG. 2 is so extreme as to sometimes not provide positive pressure past the centerline of the compressor wheel, measured in a plane perpendicular to the axis of the compressor wheel. In aerodynamic testing of a commercially available, as seen in FIG. 2, with a tight radius bend, it was seen that, the flow of air (100) at the inlet to the duct was uniform across the plane of the inlet. As the flow of inlet air (101) reaches the bend in the duct, the energy is sufficient to support attached flow around the initial radius of the bend. Further around the tight inner bend radius separation (112) of the flow is sufficiently significant that the remainder of the flow (102) does not reach the centerline of the duct.
Second, typical turbocharger compressor wheel blades are excited through several orders. For commercial turbochargers, the design criteria typically are such that the blades are designed to exclude up to the fourth order of vibration. For a reasonable pressure gradient across the inlet to the compressor wheel, this design criteria is sufficient to prevent HCF failure in the blades over a turbocharger compressor's multiple lifetimes. However when the pressure gradient across the inlet to the compressor wheel is severe, as in the case of the tested inlet ducts in FIG. 2 through 6, the excitation is sufficient to cause HCF failure in blades of compressor wheels which would otherwise be OK. In these non-symmetric pressure gradient ducts each blade of the compressor wheel sees a once-per-revolution pressure pulse ultimately leading to HCF failures.
Third, as a result of flow separation in the bend, there is a significant average pressure drop across the compressor wheel inlet. This change in inlet pressure or flow can, in the worst case, cause the compressor to go into surge, or, in a less violent case, cause a loss of pressure ratio and efficiency, as can be seen in FIG. 9.
In general, those working in this art have accepted the aerodynamic inefficiencies discussed above with resignation, using a simple elbow as shown in FIG. 2.
The problems of the simple tight bend, as seen in FIG. 2, are also seen in commercially available inlet duct bends seen in FIG. 3, FIG. 4 and FIGS. 6 A and B. In the duct bend in FIG. 4 the direction changing segment is somewhat disc-shaped, with a planar surface (32) perpendicular to the wheel axis, the increased volume of this shape allowing the axial space for the inlet bend to be compressed even more than that of the simple tight bend (31) in FIG. 3. This means that the airflow in the zone of the inner bend has to flow around an even tighter radius, resulting in flow separation (113, 114) around the inner bend and some separation (106) due to the cavity at the bottom of the bend. In this case the majority of the flow (104, 107) which reaches the compressor wheel is confined to the lower half of the wheel only.
In the arrangement seen in FIG. 6A and FIG. 6B, a configuration, which is in production, has the air cleaner in close proximity to the compressor inlet. The separated flow on the inner radius means that flow separation (116) occurs to a high degree and testing revealed that this degree of separation was sufficient to cause the turbocharger to go into premature surge, sufficient to raise the temperature of the inlet air enough to melt the plastic of the duct and even some of the media in the filter.
This problem is addressed in a direction changing duct as shown in FIG. 5A and FIG. 5B and disclosed in United States Patent Application Publication No. 20040134461 (Bishop). The duct includes a 90° bend for changing the direction of flow of compressed air being supplied from a compressor to a carburetor. To ensure delivery of the same amount of flow to the front and rear barrels of a four-barrel carburetor, and to address the problem shown in FIGS. 2, 3, 4, 6 a flow divider is provided extending diametrically through the passage from the inlet to the outlet, dividing flow into separate upper and lower channels (115). The lower channel delivers air to the front barrels and the upper channel delivers air to the rear barrels of the carburetor. However, besides the complexity of manufacturing a curved duct with an integral flow divider, and the probability of such a flow divider breaking loose or otherwise fail, there is a more significant problem. Flow velocity and pressure measured over the area of the duct outlet is not even. This may not be a problem in the Bishop environment of use, which is supply of compressed air to a carburetor, but it would be a problem in applications where a more balanced output is required, such as in the supply of air to a compressor wheel inlet. For such a supply of air to the compressor wheel inlet where slender compressor wheel blades are used, there is the increase of HCF and likelihood of failure, as discussed above.
There are many other configurations of turbocharger positioning not being conducive to acceptable aerodynamics of the inlet. This situation exists in both commercial diesel and automotive applications. On Vee engines, in ether category, the requirements of packaging often force the turbocharger to sit sideways in the valley of the engine. Because of this configuration, the compressor inlet is often cramped by the cylinder heads. In some twin turbocharger vee engine configurations the turbocharger sits in a position low outside the engine, adjacent to the front of the engine, so a tight compound bend is required from the air cleaner duct to the compressor air inlet. There is thus a need for a direction changing duct able to change the direction of flow of a fluid in a short distance and provide a greater balance of flow and pressure over the entire outlet area, and to do this with minimal pressure drop. There is a further need for such a duct that is able to increase the uniformity as described above, while providing a cost-effective and reliable component of the turbocharger system.